I find the entrance block of a National rather an inadequate design as a way of sealing up a hive before transportation - the dimension of the wood has to be exact - and it never seems to be that's if you can find one. And if the brood box was to move, then the block might well fall out - so I usually finish up taping the entrance up with gaffa tape instead. the kewl floor design means that the strip of wood to seal it up does not have to be an exact fit - it can be a bit sloppy - provided there is a way of holding it in place. Maybe a brass or stainless steel piano hinge could be used along the width of the entrance so the wood is never lost? And then the opened up entrance could act as a landing board?